Home Power Magazine - Design & Installationhttp://www.homepower.com/solar-electricity/design-installation
enBACK PAGE BASICS: Sizing Very Small PV Systemshttp://www.homepower.com/articles/solar-electricity/design-installation/back-page-basics-sizing-very-small-pv-systems
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Intermediate</li></ul></div><div class="field field-name-field-author field-type-node-reference field-label-inline clearfix"><div class="field-label">By:&nbsp;</div><div class="field-items"><div class="field-item even"><a href="/profiles/ian-woofenden">Ian Woofenden</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">Sharing solar technology with people who do not have electricity is a satisfying venture, and learning how to do it well helps us understand the basics of off-grid solar-electric system sizing. An example from my work in Costa Rica can help you grasp the process.</span></p>
<p>The three key design questions are:</p>
<p>• What is the electrical load?<br />
• What PV array capacity is needed?<br />
• What battery capacity is needed?</p>
<p>These three main parts of the system must be sized so that the load is well supplied, and the battery is regularly fully charged and never overdischarged. (A charge controller, which is not discussed here, would also be needed for this system to prevent the battery from being overcharged.) Finding that balance uses watt-hour math combined with some design experimentation and savvy.</p>
<p>Load. All off-grid PV system design starts with the load—how much energy is needed per day. Energy is measured in kilowatt-hours (kWh) in large home systems common in North America. In small developing-world systems, it’s measured in watt-hours (Wh). The typical system we install includes three to five DC LED lights and a USB outlet converter for cell phone charging.</p>
<p>Calculating array capacity is based on:</p>
<p>• Daily energy needed<br />
• Peak sun-hours on the site<br />
• Modules available</p>
<p>In this design, we had several 20-watt modules available, and 4 peak sun-hours in the region. Using a basic formula (PV watts × peak sun-hours × 0.65 “reality factor,” which accounts for energy losses of an off-grid system and an unrealistic module rating system), we end up with 52 Wh per day from our 20 W module. This is almost three times the daily load and may seem excessive—but having this sort of headroom is wise with small systems, and accommodates some load growth.</p>
<p>Battery sizing considers:</p>
<p>• Daily load<br />
• Days without sun (days of autonomy)<br />
• Recharge time with chosen PV module<br />
• Batteries available</p>
<p>When we went battery shopping, we found a 12 Ah, 12 V battery, which means its energy capacity is 144 Wh. We routinely cut this number in half to give us the usable capacity, basing our design on an average 50% depth of discharge (DOD). This leaves us with 72 Wh of usable capacity. An 18 Wh daily load will only cycle the battery down about 12.5%. If we design based on a maximum of four days without sun, we are right at 50% DOD.</p>
<p>Also important is how long it will take to fully recharge the battery after four rainy days. With our “extra” PV charging capacity included in our 52 Wh per day, and assuming we continue to use the load at the full 18 Wh per day, it will take just over two days to catch up and fully recharge the battery (52 - 18 = 34 Wh extra per day; 72 ÷ 34 = 2.1 days). And this all assumes that the load doesn’t grow, which is not always a safe assumption.</p>
<p>Finding the balance between the electrical load, solar generation, and battery capacity is a bit of math and a bit of art based on experience, and must take into consideration the real conditions at the site and product availability. Understanding the basic questions to ask will get you off to a good start for your own solar-electric system.</p>
</div></div></div>Tue, 30 Jun 2015 19:17:04 +0000Michael Welch13466 at http://www.homepower.comhttp://www.homepower.com/articles/solar-electricity/design-installation/back-page-basics-sizing-very-small-pv-systems#commentsPV Disconnecting Meanshttp://www.homepower.com/articles/solar-electricity/design-installation/pv-disconnecting-means
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Advanced</li></ul></div><div class="field field-name-field-author field-type-node-reference field-label-inline clearfix"><div class="field-label">By:&nbsp;</div><div class="field-items"><div class="field-item even"><a href="/profiles/ryan-mayfield">Ryan Mayfield</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">In the National Electrical Code (NEC), methods for disconnecting PV systems are covered in Part III of Article 690, “Disconnecting Means.” The sections contained within Part III received a fair amount of updating in the 2014 NEC. Some of these updates were merely relocations from different sections in the 2011 version and some, specifically 690.17, are bonafide changes.</span></p>
<p>Section 690.13 lays out the general requirements for disconnecting means for a PV system’s ungrounded DC conductors. The first subsection, 690.13(A), states that the “PV disconnecting means shall be installed at a readily accessible location either on the outside of a building or structure or inside nearest the point of entrance of the system conductors.” By itself, this rule would require nearly all rooftop PV systems to keep the PV circuits on the building exterior until they reach a readily accessible disconnect. Generally, rooftops are not considered readily accessible nor are the spaces directly below a rooftop, such as an attic, which is commonly the nearest point of entrance.</p>
<p>The exception that follows 690.13(A) allows the PV circuits to run inside a building, provided the conductors are installed to meet 690.31(G). [Note: In the 2014 NEC, a typo in the exception points to the wrong section—690.31(F).] Section 690.31(G) allows DC circuits to run inside any building, provided they are installed in metallic raceway or use specific metal-clad cable. This section is specific to the DC conductors, so does not apply to inverter output circuits (as with rooftop microinverters). The section title: “Direct-Current Photovoltaic Source and Direct-Current Output Circuits on or Inside a Building,” has been modified in the last two NEC cycles to help clarify the application, since many authorities having jurisdiction (AHJs) were also applying these rules to AC circuits.</p>
<p>Section 690.13(B) requires permanently marking the disconnecting means. No specifics are given; you have to look at 690.17(E) and 690.53 for those details. 690.13(C) states that PV system disconnecting means do not need to be designated as “service equipment” (which is required for service conductors to a building), another point some AHJs have mistakenly tried to require.</p>
<p>The last two subsections in 690.13 cover the number and grouping of all the system disconnects. In short, there cannot be more than six disconnects per PV system and the disconnects need to be grouped together. The limit of six disconnects can seem troubling at first, especially for installations that use multiple inverters. But revisit Article 100, which defines PV systems as all of the components required to convert sunlight to electrical energy. Therefore, in a system with multiple inverters, each inverter could have up to six disconnecting means on the DC side.</p>
<p>Section 690.15 details the equipment that requires disconnecting means: inverters, batteries, and charge controllers. If the equipment is energized from more than one source, the disconnecting means for the equipment needs to be grouped together. In a grid-tied system, this requires placing the DC and AC disconnecting means for an inverter, for example, in proximity to each other. A properly rated circuit breaker can act as the disconnecting means, so for many installations, the point of utility interconnection will satisfy the disconnecting means as well (for example, the backfed circuit breaker for a load-side connection).</p>
<p>This section on disconnecting means isn’t without interpretation problems, as the last sentence of 690.15 and a provision in the next section show. The final sentence in 690.15 states that “a single disconnecting means in accordance with 690.17 shall be permitted for the combined AC output of one or more inverters or AC modules in an interactive system.” In the 2011 NEC, this reference to 690.17 was less contentious than in the 2014 version. The reason is, in 2014, the acceptable disconnect types are all DC disconnecting means. Therefore, the NEC is requiring a DC-rated switch for an AC source. As previous NEC cycles show, the intention was to require an approved disconnecting means be used for multiple inverters.</p>
<p>Section 690.15(A), “Utility-Interactive Inverters Mounted in Not Readily Accessible Locations,” was simply moved from 690.14 into 690.15. Other than abbreviations, the language did not change. In short, utility-interactive inverters can be located in not readily accessible locations, such as a rooftop, as long as they meet these requirements:</p>
<p>• Have AC and DC disconnecting means “within sight of or in each inverter.”<br />
• Install an AC disconnect in a readily accessible location per 690.13(A).</p>
<p>The final requirement in the “not readily accessible inverter” section is a plaque at the service entrance and points of utility interconnection that denotes the presence of the PV system. These can be combined with other required plaques to meet multiple NEC sections simultaneously.</p>
<p><strong>Microinverter Disconnects</strong></p>
<p>With the addition of the rapid shutdown requirement (see “Code Corner” in HP165 and HP166), many more commercial systems are likely to have inverters on rooftops, requiring installers to pay attention to requirements in 690.15(A). And while this NEC section is indiscriminate when it comes to inverter types, it often leads to another question specific to microinverters: Can connectors be used as disconnects?</p>
<p>To get a clear understanding, we need to jump to the exception in 690.17(E), which allows connectors to be used as disconnecting means, but only if they comply with 690.33, and are listed and identified for use with the specific equipment. Section 690.33(E) specifies the requirements for circuit interruption using connectors: They either need a rating for interrupting the current without hazard to the user or have a marking on them indicating “Do Not Disconnect Under Load” or “Not for Current Interrupting.”</p>
<p>This requires verifying the manufacturer’s product listing. The major microinverter manufacturers have listed their products with specific connectors for disconnecting means and provide technical documentation to help AHJs and installers with that evaluation. The final say rests with that AHJ, so the burden of proof often falls back to the installer. In jurisdictions that do not accept the connectors as disconnects, installing a switch at the end of the microinverter’s branch circuit can provide a disconnecting means for an entire group of modules and inverters.</p>
</div></div></div>Tue, 30 Jun 2015 18:51:16 +0000Michael Welch13464 at http://www.homepower.comhttp://www.homepower.com/articles/solar-electricity/design-installation/pv-disconnecting-means#commentsAdding Battery Backup to Your PV System with AC-Couplinghttp://www.homepower.com/articles/solar-electricity/design-installation/adding-battery-backup-your-pv-system-ac-coupling
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Intermediate</li></ul></div><div class="field field-name-field-author field-type-node-reference field-label-inline clearfix"><div class="field-label">By:&nbsp;</div><div class="field-items"><div class="field-item even"><a href="/profiles/justine-sanchez">Justine Sanchez</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">Batteryless grid-tied PV (GT PV) systems are affordable, efficient, and simpler than their battery-based counterparts. But when the grid goes down, the system goes offline, leaving the homeowner without electricity. Fortunately, there are solutions for those who don’t want their electricity access disrupted by utility outages.</span></p>
<p>There are two different approaches to integrating battery backup into an existing GT PV system—DC-coupling or AC-coupling. Both involve adding a battery bank and a battery-based inverter-charger; plus the associated disconnects and overcurrent protection, and a backed-up (or “critical”) loads subpanel for the appliances that need to continue operating during an outage.</p>
<p><strong>The DC Approach</strong></p>
<p>The conventional battery-based PV system is “DC-coupled”—all power generation is on the DC side of the system. All sources operate at the same system input voltage, typically 12 to 48 VDC.</p>
<p>This strategy avoids the compatibility, technical, or warranty issues of an AC-coupled system (see below). Benefits include having the preferred three-stage (tapered off) battery charge control during all conditions and being able to recharge the batteries during a utility outage, even after they have been drained beyond the low-voltage cut-out point.</p>
<p>To minimize wire size, which is more economical for long wire runs, PV arrays can be wired for higher DC voltage (commonly 150 VDC but up to 600 VDC) using a step-down charge controller to convert to battery bank voltage. Energy stored in the battery bank is converted to AC by an inverter–charger, which is usually set up to provide energy first to a critical loads sub-panel, then to the grid through the main distribution panel and utility meter. In the event of a grid failure, the critical loads will continue to be powered by the inverter.</p>
<p>DC-coupling a grid-tied system can take two approaches. Both replace the existing batteryless inverter with a battery-based inverter-charger.</p>
<p>The first approach adds a standard, low-voltage (150 VDC maximum) charge controller. Depending on the number and size of the PV modules, adding or eliminating PV modules may be necessary to meet the input voltage requirements of the charge controller. The PV array’s wire size may also need to be increased (lower voltage means higher amperage, necessitating larger wire to carry the current).</p>
<p>The existing high-voltage (up to 600 VDC) PV array will have to be reconfigured for the charge controller’s DC input voltage range. A combiner box and overcurrent protection (fuses or circuit breakers) may need to be installed if there are three or more paralleled series strings.</p>
<p>The second DC-coupling approach uses a higher-voltage (up to 600 VDC) charge controller. This method eliminates rewiring the array and adding a combiner box. While you may save labor and equipment costs, the more expensive high-voltage charge controller may negate these savings. Lower-voltage charge controllers can be about $1,000 less than their higher-voltage counterparts.</p>
<p><strong>AC-Coupling</strong></p>
<p>AC-coupling has been used for creating stand-alone “microgrids” in remote areas for many years. Now it is gaining wider industry acceptance as an option to retrofit battery backup into an existing batteryless system. There are unique challenges to AC-coupling, however.</p>
<p>The AC-coupling approach combines the AC outputs of all available sources (whether grid, generator, or RE-powered inverters). When connected to the grid, the batteryless grid-tied inverter feeds the PV array-generated energy into a critical loads panel. A separate inverter–charger is connected to the battery bank that maintains the battery bank voltage and allows AC power to pass through, either from or to the grid.</p>
<p>If there isn’t enough PV energy to supply all the critical loads, the inverter-charger adds grid energy. Alternately, any excess energy not used by the backed-up loads is fed back through the inverter–charger into the main distribution panel. If solar-electric supply exceeds household electrical demand, then the excess energy flows to the grid.</p>
<p>In the event of a utility outage, an internal isolation relay in the inverter-charger disconnects it from the grid, and supplies the backed-up load center from the batteries. During grid failures, after the batteryless grid-tied inverter senses that the inverter–charger has begun to provide consistent power (typically after a 5-minute delay), it synchronizes with the inverter-charger and begins to also feed the backed-up load center from the PV array. The inverter-charger effectively “tricks” the batteryless inverter into feeding its power into a “new” grid established by the inverter-charger.</p>
<p>If the PV array output of the batteryless inverter is connected to the main distribution panel instead of the backed-up load center, the energy from the array and batteryless inverter cannot be used by the backed-up power system because during a grid outage it becomes isolated and will not have AC line-voltage present, which it needs to work. So, when retrofitting to an AC-coupled system with battery backup, the batteryless inverter output circuit will have to be moved to a new specific backed-up load panel, along with the household circuits that you want to operate during an outage. Relocating those household circuits to a subpanel needs to happen in any retrofit, regardless if the system is AC- or DC-coupled. The inverter–charger can charge batteries from either the PV array or the utility if available. Additionally, an engine-generator can be added to the system, with all power generation sources controlled by the inverter-charger.</p>
<p>This approach offers several advantages. First, you can continue using your existing batteryless inverter. Also, there’s no need to purchase a separate charge controller or combiner box, and the array doesn’t have to be rewired.</p>
<p>Similar to DC-coupled systems, an inverter-charger is added to the system. Note, the power rating of the batteryless inverter usually cannot exceed the capacity of the inverter-charger (and in some cases, needs to be 10% to 25% less). You have to consider how excess PV array energy is handled by the inverter-charger when the grid is down, and also how to deal with a system that has shut down, should low battery cutout voltage be reached during a utility outage. And finally, potential equipment warranty and compatibility issues need to be addressed. The above AC-coupling issues are covered in detail below.</p>
<p>The primary benefits of AC-coupling may include higher system efficiency (especially when the AC loads are used concurrently with PV array production); the ability to use smaller array wire; and fewer DC components. Drawbacks may include lower system efficiency if energy demand doesn’t align with PV production (efficiency losses from two inverters) and in some cases higher system costs.</p>
<p><strong>The Complexities of AC-Coupling</strong></p>
<p>Surplus energy. One challenge with an AC-coupled system is surplus energy produced by the PV array during a utility outage, which needs to be controlled to avoid overcharging the batteries. Some battery-based inverter-chargers can control the batteryless inverter, throttling back its output and/or use an AC frequency shift, which can trip the batteryless inverter offline. Other inverter-chargers use relays to shut down the batteryless inverter. However, unless the inverter-charger can throttle back PV array power, these options do not provide the preferred tapered charge controlling—they only shut the PV power fully on or off, which is not the best charging strategy for batteries. Note: There is also the possibility of using a diversion load and diversion controller, which may also offer tapered charge control.</p>
<p>Low battery voltage. Without a backup generator or the ability to reduce the energy consumption of the backed-up loads (load shedding), low battery bank voltage during a grid outage may trigger the inverter-charger to go offline. This eliminates the batteryless inverter’s ability to recharge batteries from a PV source, since it depends on the inverter-charger’s continued output to stay online.</p>
<p>Here’s an example: A grid outage occurs at noon on a cloudy day. To keep loads running, the inverter-charger pulls energy primarily from the battery bank through the day and into the night. The next morning, the battery bank is below the inverter-charger’s low-voltage cutout point, triggering a shutdown. Although the sun is now shining, both inverters are offline and nonfunctional, and the PV source is unusable until the grid is functional again and the batteryless inverter starts up. However, in some cases the inverter–charger needs to be manually restarted before the system will operate.</p>
<p>Equipment compatibility. While there are a few battery-based inverter manufacturers (SMA America and Schneider Electric) that make both batteryless and battery-based inverters, in many cases, retrofitting an existing GT system means pairing different inverter brands. You will need to research if using particular equipment in an AC-coupled system could void any existing equipment warranties.</p>
<p>Some PV batteryless inverters use a “grid-impedance check” to verify that the grid can receive energy and cannot be “tricked” by the higher impedance output of some inverter-chargers. Also, the AC output voltage of the batteryless inverter and the inverter-charger must match. For example, unless your inverter-charger offers split-phase 120/240 VAC output, you will either need to stack two 120 VAC inverter-chargers or use a step-up/down transformer to match the output of the 240 VAC batteryless inverter.</p>
<p>AC engine-generators can be included in an AC-coupled system, but precautions (for example, interlocked disconnects) must be taken to make sure the generator and batteryless inverter cannot backfeed each other, which could result in damaged equipment.</p>
<p><strong>What Will It Cost?</strong></p>
<p>Cost factors include whether an array needs to be reconfigured and distance from the array to the batteries. The array size, critical load profile, how long backup is required, and whether a generator is included determines battery bank and battery-based inverter-charger requirements. Will you use a pre-wired power panel that includes all the required disconnects, overcurrent protection, metering, etc., needed to incorporate the battery-based inverter-charger or will you piece this gear together yourself?</p>
<p>Given all these variables, it is tough to come to a general cost comparison. But, out of curiosity, we examined the costs of adding batteries to a 3.5 kW batteryless system using three strategies: low-voltage DC-coupling, high-voltage DC-coupling, and AC-coupling. And for this particular example, we found that the options were within about $500 of each other. Using AC-coupling equipment was slightly less expensive—but any option is a significant investment. The equipment cost of adding battery backup via AC-coupling to this system (19 kWh of AGM battery storage, a 4,000-watt battery-based inverter-charger; and a pre-wired power center) was more than $10,000—using MSRP pricing and without including a battery box, battery cables, shipping, taxes, or labor.</p>
<p>As they say, “hindsight is 20/20”—a standard DC-coupled option would likely be less expensive than AC-coupling if the system was designed with battery backup from the get-go, since you only need the inverter-charger (and not the batteryless inverter). Additionally, when that equipment is part of a new PV installation, the additional cost (due to batteries, charge controller, associated DC disconnect gear, and installation) could be partially offset by the 30% federal renewable energy tax credit.</p>
<p>Beyond the cost are the details: Significant damage to both equipment and personal injury could result if the equipment is not installed or used properly. Equipment must be installed and operated in accordance with manufacturer instructions and the National Electrical Code.</p>
<p>[Special thanks to Jsun Mills, who provided the original insight and information that this article stemmed from.]</p>
</div></div></div>Mon, 29 Jun 2015 22:48:51 +0000Michael Welch13452 at http://www.homepower.comhttp://www.homepower.com/articles/solar-electricity/design-installation/adding-battery-backup-your-pv-system-ac-coupling#commentsASK THE EXPERTS: Cleaning PV Moduleshttp://www.homepower.com/articles/solar-electricity/design-installation/ask-experts-cleaning-pv-modules
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Beginner</li></ul></div><div class="field field-name-field-author field-type-node-reference field-label-inline clearfix"><div class="field-label">By:&nbsp;</div><div class="field-items"><div class="field-item even"><a href="/profiles/christopher-laforge">Christopher LaForge</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>My PV array is three years old and needs cleaning—I can see dirt/film on the modules and we’ve not had enough rain to wash them off. Plus, module output is down, according to the inverter’s data. What chemicals or detergents can I use to clean the array?</p>
<p>Dan White • Cat Spring, Texas</p>
<p>Cleaning modules in large commercial arrays is done based on a cost-benefit analysis that compares cleaning costs with the revenue increase that results from the improved array output. Most residential and small commercial users want their array to perform optimally all the time, and may therefore clean them more often than commercial arrays are cleaned.</p>
<p>Either way, cleaning should be done per manufacturer’s instructions. For example, <em>SolarWorld</em>’s publication, <em>Quick Guide for Users</em>, states: “Given a sufficient tilt (at least 15°), it is generally not necessary to clean the modules (rainfall will have a self-cleaning effect). In case of heavy soiling, we recommend cleaning the modules using plenty of water (from a hose), without any cleaning agents and using a gentle cleaning implement (a sponge). Dirt must never be scraped or rubbed away when dry, as this may cause micro-scratches.”</p>
<p>Cleaning arrays can be dangerous. Clean the modules with a hose and a soft cloth if you can access them from the ground. Do this only when the modules are cool—early morning, on cloudy days, or in the evening after they have cooled. If you need to climb a structure to access the modules, use appropriate safety gear—full-body harnesses adequately secured are OSHA requirements when working more than 6 feet off the ground.</p>
<p>If you have hard water, be sure to squeegee off the rinse water so you don’t leave mineral deposits on the glass. If you have oily or greasy stains on the modules (these can occur if the array is near roads or airports), isopropyl alcohol can be used to spot-clean stained areas. Most manufacturers do not recommend using anything other than water for general cleaning; do not use soaps, solvents, or other cleaning products.</p>
<p>Many people use pressure washers to clean their arrays. Manufacturers that include instructions for using pressure washers indicate that the pressure used should be less than 80 psi.</p>
<p>For reaching large arrays and roof-mounted PV modules, lightweight telescopic poles are available with squeegee and other cleaning attachments. Novel cleaning products claim to help keep modules from getting dirty, but with water working for most cleaning and alcohol available for tough stains, additional products are not necessary.</p>
<p>If you are cleaning your modules because of output degradation, it is a good idea to check the electrical connections as well. It’s also a great time to check the mechanical connections.</p>
<p>Christopher LaForge • Great Northern Solar</p>
</div></div></div>Thu, 25 Jun 2015 02:02:46 +0000Michael Welch13443 at http://www.homepower.comhttp://www.homepower.com/articles/solar-electricity/design-installation/ask-experts-cleaning-pv-modules#commentsASK THE EXPERTS: Module Efficiencyhttp://www.homepower.com/articles/solar-electricity/equipment-products/ask-experts-module-efficiency
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Intermediate</li></ul></div><div class="field field-name-field-author field-type-node-reference field-label-inline clearfix"><div class="field-label">By:&nbsp;</div><div class="field-items"><div class="field-item even"><a href="/profiles/zeke-yewdall">Zeke Yewdall</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Where can I buy PV cells with a higher efficiency than 18%? I’m searching online, but I have not been able to find anything.</p>
<p>Raymond Collins • via email</p>
<p>We’re used to looking at the efficiency of appliances, and generally, the more efficient the appliance is, the better. But for solar-electric modules, that is not necessarily the case. While that 18% efficient module might look pretty slick, it might not be the best for your situation, and may be appropriate only when mounting space is limited. Let’s look at some possibilities:</p>
<p>The cheapest option is not using the least-expensive and least-efficient modules. In fact, those modules have the highest installation cost (more modules, more rack, and more installation time per rated watt). In this scenario, unless space is at a premium, it’s also not worth it to buy the highest-efficiency modules—they actually cost more to get the same amount of energy than either of the less-efficient options.</p>
<p>Another assumption that many people have—incorrectly, much of the time—is that they should install the highest-wattage modules available on their roof. For example, one installer might bid with 280 W modules, but another specifies 320 W modules. A higher-wattage PV module may or may not be suitable for your project. Those 320 W modules might also have a higher efficiency (reducing array size, but perhaps increasing array cost). Or maybe they have the same efficiency, but are just a larger module (possibly increasing labor costs due to being heavier, or reducing labor costs due to fewer modules being needed).</p>
<p>And at the end of the day, the number that you, as a homeowner, care about is usually how much will it cost to fit the amount of rated power you want on your roof (or in your yard). This number is affected by the efficiency of the modules, but there are a lot of other variables that go into arriving at that final number, too, so don’t get hung up on looking just at the efficiency or power rating of the modules.</p>
<p>Zeke Yewdall • Mile Hi Solar</p>
</div></div></div>Thu, 25 Jun 2015 01:34:55 +0000Michael Welch13441 at http://www.homepower.comhttp://www.homepower.com/articles/solar-electricity/equipment-products/ask-experts-module-efficiency#commentsMAILBOX: Grounding Conceptshttp://www.homepower.com/articles/solar-electricity/design-installation/mailbox-grounding-concepts
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Advanced</li></ul></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">On page 37 of <em>HP166 </em>, there is a highlighted box talking about equipment grounding. An important distinction needs to be pointed out: Electricity does not go to earth, it goes back to its source. </span>Yes, equipment grounds are bonded to the grounding electrode at the service entrance, but they are also bonded to the grounded conductor or neutral at the service entrance via the main bonding jumper. The grounding electrode or ground rod in this case serves a number of purposes, but it is not to clear a ground fault.</p>
<p>When a ground fault occurs, it gets back to its source through an effective ground-fault current path (properly done equipment grounding), through the neutral at the service to its source, and then trips the breaker on the faulted circuit, thus stopping flow of electricity and preventing non-current-carrying metal parts from being a shock hazard. Earth is never an effective ground-fault current path. Even if a ground rod has 25-ohm resistance to ground (which is rare), you would get 4.8 A—nowhere near enough to open a circuit breaker on a 15 A branch circuit to clear a fault.</p>
<p>In the case mentioned in the article, on the DC side, all non-current-carrying metal parts on the array would still be a shock hazard, even if the inverter were turned off by the ground fault, until a technician finds and fixes the fault on the array. I think these distinctions are important to point out. Thanks for a great magazine—I enjoy it immensely.</p>
<p>Steven Johnson • Tucson, Arizona</p>
<p>Thanks so much for your letter. You are absolutely correct. The last part of that sidebar should have instead stated: “Equipment grounding ensures metallic objects in an electrical system are at the same voltage potential (earth) and thus is a safety measure to reduce shock hazards. Additionally, should a ground fault occur—unintentional electricity flowing, for example, to a module frame due to a nicked wire— the electricity has a pathway back to the ground-fault protection device (GFPD) so it can interrupt the flow of fault current and keep the system non-operational until the source of the fault has been corrected.” The html version on our website and the downloadable PDF have been corrected.</p>
<p>Justine Sanchez • <em>Home Power</em> Senior Technical Editor</p>
</div></div></div>Thu, 25 Jun 2015 01:13:33 +0000Michael Welch13439 at http://www.homepower.comhttp://www.homepower.com/articles/solar-electricity/design-installation/mailbox-grounding-concepts#commentsMAILBOX: Smaller Systemshttp://www.homepower.com/articles/solar-electricity/design-installation/mailbox-smaller-systems
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Beginner</li></ul></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">I read with interest the “Mailbox” letter from Paul Hoover in<em> HP164</em>. We are happy off-gridders since 1983 and <em>HP1</em>. </span>Our independent (battery-based) PV system began with one 35 W module and grew to six 60 W and two 55 W (for water pumping) modules on a tracker. When we sold our first homestead and moved to our new home, we figured we were way overplanning by having twelve 110 W modules on a tracker! Yet, that system is dinky by today’s standards, and even now becoming a bit tight with our own energy use.</p>
<p>But to agree with Paul, living with a smaller system has its own built-in blessings. It’s self-educating regarding conservation. And it saves one’s budget and cost outlay requirements.</p>
<p>The only drawback we have encountered—and only recently—is the challenge of expanding our system. Way back, most systems looked more like the early versions of<em> Home Power</em>’s “Democracy Rack,” with its three of this, five of that, and couple of another mix of modules. What we are now learning is that mixing different wattages of modules isn’t a good idea, since it seems to make all the modules operate at the power of the lowest-rated module. So if we are to add to our current system, we either have to find more 110-watt modules—which appear to be non-existent—or run separate wires from new modules to a separate charge controller. This is not an easy task for relatively small gain, due to distance and the need for buried conduit. And good luck with finding someone to sell you a “few odd modules”!</p>
<p>It’s this type of unanticipated surprise that has been put on the plates of off-gridders, and yet we have adjusted and grown over time. Again, I agree with Paul that small stand-alone systems are not as appreciated now that grid-tied systems have finally come into their own.</p>
<p>Katcha Sanderson • Scott Valley, California</p>
</div></div></div>Thu, 25 Jun 2015 00:19:14 +0000Michael Welch13437 at http://www.homepower.comhttp://www.homepower.com/articles/solar-electricity/design-installation/mailbox-smaller-systems#commentsGEAR: MT Solar Pole Mounthttp://www.homepower.com/articles/solar-electricity/equipment-products/gear-mt-solar-pole-mount
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Intermediate</li></ul></div><div class="field field-name-field-author field-type-node-reference field-label-inline clearfix"><div class="field-label">By:&nbsp;</div><div class="field-items"><div class="field-item even"><a href="/profiles/justine-sanchez">Justine Sanchez</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">MT Solar (<a href="http://polemount.solar" target="_blank">polemount.solar</a>) offers a pole-mount system that is assembled at waist-level, then chain-hoisted to the desired height. </span>This system is offered as a single pole mount (for two to 12 modules) or as a multipole system (stacking modules in a landscape orientation, three to four high). The pole mount uses top-clamp mounting hardware and heavy-duty rails from other solar racking companies. (MT Solar can supply rails if needed). The array tilt can be adjusted with a hand crank that’s accessible from ground level.</p>
</div></div></div>Wed, 24 Jun 2015 23:52:37 +0000Michael Welch13432 at http://www.homepower.comhttp://www.homepower.com/articles/solar-electricity/equipment-products/gear-mt-solar-pole-mount#commentsFROM THE CREW: Off-Grid, On-Grid, or Somewhere In Betweenhttp://www.homepower.com/articles/solar-electricity/design-installation/crew-grid-grid-or-somewhere-between
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Beginner</li></ul></div><div class="field field-name-field-author field-type-node-reference field-label-inline clearfix"><div class="field-label">By:&nbsp;</div><div class="field-items"><div class="field-item even"><a href="/profiles/michael-welch">Michael Welch</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">In a few, more-expensive utility districts, a milestone with its own buzzword has recently been reached: grid parity. That’s the time when the cost of making your own electricity with a grid-tied PV system becomes as cheap—or cheaper—than what the utility charges. Without batteries, grid-tied system costs are low enough to justify rooftop solar electricity purely on an economic basis.</span></p>
<p>However, as grid-tied PV systems are becoming more popular, some utilities are starting to penalize them—charging higher electricity rates and higher monthly connection fees, and supporting legislation attempting to hobble net-metering programs. To some customers, grid defection—leaving the grid by installing batteries and a battery-based inverter with a PV array—is looking more attractive.</p>
<p>Some say the utilities are worried about grid defection—if it takes off, their economic hit could be huge. Others say that a critical mass of disconnecting customers will never happen—Americans are too accustomed to the seemingly endless electricity in a nearly effortless grid.</p>
<p>While significant grid defection isn’t likely to happen anytime soon—it’s too expensive to justify in most locations—historically, Home Power readers have been early adopters in making changes. Inexpensive, more user-friendly batteries and the continued decreasing cost of PV systems may help drive change, too.</p>
<p>Instead of total defection, some utility customers are considering a middle road—load defection, which some folks define as picking and choosing appliances within the home to remove from grid power. This tactic is similar to the old “Take Your Bedroom Off the Grid” concept (<em>HP60</em> and <em>HP73</em>), where homeowners would isolate electrical circuits from the grid and install a stand-alone battery-based system to power those circuits, usually with a PV system.</p>
<p>Another, larger-scale means of defection is the concept of neighborhood microgrids. The technology is already developed that, in the future, could allow neighborhoods to disconnect from their existing energy providers and create localized energy distribution systems, with PV arrays on every rooftop and energy storage strategically distributed throughout the microgrid. Rules to separate electricity distribution ownership from centralized energy production are being considered by utilities and regulators, and what they allow and disallow will be key to the implementation of microgrids within utility territories.</p>
</div></div></div>Wed, 24 Jun 2015 23:32:03 +0000Michael Welch13430 at http://www.homepower.comhttp://www.homepower.com/articles/solar-electricity/design-installation/crew-grid-grid-or-somewhere-between#commentsASK THE EXPERTS: Battery Box Coatinghttp://www.homepower.com/articles/solar-electricity/equipment-products/ask-experts-battery-box-coating
<div class="field field-name-field-skill-level field-type-taxonomy-term-reference field-label-hidden clearfix"><ul class="links"><li class="taxonomy-term-reference-0">Intermediate</li></ul></div><div class="field field-name-field-author field-type-node-reference field-label-inline clearfix"><div class="field-label">By:&nbsp;</div><div class="field-items"><div class="field-item even"><a href="/profiles/allan-sindelar">Allan Sindelar</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span class="lead-in">I just finished reading Allan Sindelar’s “Battery Box Design”</span> article in <em>HP141</em> (<a href="http://homepower.com/141.96" target="_blank">homepower.com/141.96</a>). He suggests that a simple battery box can be built from plywood, but recommends lining the interior and also using a penetrating oil or paint to help the wood resist minor spills. I’m wondering if he has any specific brand recommendations for coating the plywood?</p>
<p>Luis Pérez • via email</p>
<p>I don’t have particular coating brands to suggest, as it’s not that critical. You just need a barrier between any spilled, splattered, or leaked sulfuric acid electrolyte and the plywood, which will rapidly deteriorate on contact with the acid. I have used clear polyurethane and latex-primer finishes, in both oil- and water-based forms. The water-based formulas generally have fewer volatile organic compounds (VOCs) and are easier to clean up (both from your skin and brushes). All work well for the inside of the box, as the EPDM or PVC pond-liner material serves as the heavy-duty barrier. On the exterior, your choice has more to do with aesthetics: a clear finish on higher-grade plywood can turn the box into attractive cabinetry.</p>
<p>Allan Sindelar • SindelarSolar.com</p>
</div></div></div>Tue, 28 Apr 2015 21:54:01 +0000Michael Welch13357 at http://www.homepower.comhttp://www.homepower.com/articles/solar-electricity/equipment-products/ask-experts-battery-box-coating#comments